The present invention relates generally to satellite broadcasting techniques, and more particularly, to satellite broadcasting techniques based on Code Division Multiple Access (CDMA) technology.
Satellite broadcasting systems for transmitting programming content have become increasingly popular in many parts of the world. Direct Broadcasting Satellite (DBS) systems transmit television programming content, for example, to a geo-synchronous satellite, which broadcasts the content back to the customers. In such a wireless broadcast environment, the transmitted programming can be received by anyone with an appropriate receiver, such as an antenna or a satellite dish.
In addition, a number of satellite broadcasting systems have been proposed or suggested for broadcasting audio programming content from geo-synchronous satellites to customers in a large coverage area, such as the continental United States. Satellite broadcasting systems for television and radio content provide potentially national coverage areas, and thus improve over conventional terrestrial television stations and AM/FM radio stations that provide only regional coverage.
Code division multiple access (CDMA) techniques have been proposed for satellite broadcasting systems to permit a number of programming channels to be transmitted on the same carrier frequency. Code division multiple access techniques transmit multiple information signals on the same carrier frequency, and differentiate each programming channel by encoding the channel with a unique orthogonal code.
CD Radio Incorporated has proposed a satellite broadcasting system having two satellites and a group of repeaters to provide audio service. The CD Radio system is described, for example, in U.S. Pat. Nos. 5,278,863, 5,319,673, 5,485,485 and 5,592,471. In a developed area, the direct line of sight between a mobile receiver and the transmitters on the satellites and repeaters can be blocked, for example, by underpasses or other structures. Thus, many satellite broadcasting systems transmit a delayed version of each program channel with the on-time version of the program channel to permit uninterrupted reception in the event of a blockage.
In one embodiment of the CD Radio system, for example, each satellite occupies the available bandwidth and each satellite will transmit either the on-time digital audio signal, or a delayed version of the same information for diversity purposes. While the CD Radio system does provide second order diversity, since the same information signal is received from both satellites (ignoring the effects of multi-path propagation), additional diversity gains are desirable. In a fading channel, such as the wireless channels of satellite broadcast systems, diversity has a significant impact on performance. In addition, the receivers in the CD Radio system require a unique pseudo-noise sequence for each satellite and the terrestrial repeaters to differentiate the signals from each source, adding complexity and cost to the receiver design.
Generally, a CDM satellite transmission system is disclosed that broadcasts programming content, such as audio and video information, using two geo-synchronous satellites and a plurality of terrestrial repeaters based on Code division multiple access technology. A plurality of channels are multiplexed onto a carrier frequency using Code division multiple access technology. The terrestrial repeaters operate in areas where the direct line of sight between the satellites and the mobile receiver can be blocked. Even in the presence of terrestrial repeaters, the direct line of sight between the mobile receiver and the transmitters can be blocked by underpasses or other structures. Thus, the disclosed CDM satellite transmission system transmits a delayed version of the signal with the on-time version of the signal to accommodate uninterrupted reception in the event of such a blockage.
According to one aspect of the invention, the on-time and delayed version of each information channel is transmitted by each of the satellites and repeaters, providing additional diversity gains. Thus, the satellites and terrestrial repeaters occupy the entire available bandwidth. According to another aspect of the invention, CDM transmissions are used where the pseudo-noise sequences are the same, or linear translates of one another (a delayed version to account for propagation delays), to thereby permit a more simplified receiver design.
The link between each repeater and at least one satellite is designed to be line of sight (the repeaters are positioned such that the signal can be received from at least one satellite with no blockage, or the repeater can receive a signal from a terrestrial link). Therefore, the transmissions from both satellites do not need to be repeated. Thus, in one embodiment, the repeater repeats the transmission of only one satellite.